Gonadotropin releasing hormone (GnRH) containing neurons in the primate medial basal hypothalamus (MBH), and their relationship to other peptidergic, catecholaminergic, and estrogen-synthesizing neurons in the MBH and median eminence (ME) of the monkey and rat are investigated. 1. Apparent contacts between GnRH neurons in the baboon MBH and its ventral hypothalamic tract (VHT) are identified by immunocytochemical (ICC) staining on vibratome sections, observed with light microscopy (LM), and then examined with electron microscopy (EM). Evidence for electrotonic (gap) junctions between MBH neurons is obtained using retrograde Lucifer Yellow (LY) labelling in the stalk-sectioned rhesus monkey. LY fluorescent, dye-coupled cells are ICC stained for GnRH using the PAP technique, and examined by EM for gap junctions. Other evidence for gap junctions is sought using freeze fracture of the primate VHT. 2) Cell bodies of origin of the VHT and tuberoinfundibular pathways are identified with retrograde True Blue (TB) labelling in the stalksectioned rhesus monkey. TB fluorescent cells are stained by ICC to determine the GnRH component. Results with TB are compared to those with LY, and the cell body distribution mapped. 3) Localization of aromatase (ARO) in the rat and baboon brain is determined using ICC for specific ARO enzyme subunits at the LM level. Special attention is given to the medial preoptic area, the location of the sexually dimorphic nucleus (SDN-POA) in rats, and the MBH, the two brain regions with highest ARO activity. If the rat SDN-POA is ARO-positive, we will ICC stain for ARO to identify the equivalent of the SDN-POA in the baboon. 4) Relationships between receptors for dopamine (DA), b-endorphin (END), and prolactin (PRL), and the ME nerve terminals on which they occur is investigated. Using ME synaptosomes, DA, END, PRL receptors are identified with ICC and GaRIgG-colloidal gold. The content of labelled synaptosomes is determined using ICC for GnRH, END, or tyrosine hydroxylasle (TH, as a marker for DA neurons) and the PAP technique. 5) Interactions between GnRH neurons and others belived to affect GnRH secretion are examined in the rat and baboon MBH. Dual-label ICC is performed for GnRH followed by ICC for END, ARO, or TH. Interactions identified by LM are examined by EM. This information regarding GnRH neuronal systems, interconnections, and afferent input will enhance our knowledge and understanding of neural control of reproduction in primates, including humans.